Pr. Curtis makes the point in his post below that we need to look at goals and means separately. His position is that the goal is not always sin, but certain means are always sin. What this introduces is the topic of casuistry, the application of ethical principles.
In Lutheran (i.e. Christian) ethics, the first principle must always be that we are incapable of fulfilling ANY of God's laws. There is no sinless way out for us. In this understanding, we must often "choose the lesser evil" and throw ourselves on God's mercy. This is called the theory of conflicting absolutes. There are no exceptions to God's Law. I suggest reading Norman Geisler's Christian Ethics: Options and Issues, (Grand Rapids: Baker Books, 1989); and Helmet Thielicke's Theological Ethics, ed. William H. Lazareth, vol. 1 (Philadelphia: Fortress, 1966, out of print). I have both these works and have found the knowledge contained therein to be invaluable in considering ethical issues.
Abortion is always wrong. There are no exceptions. But tubal pregnancies must be aborted or the mother will very likely die, and the baby will die in either case (tubal pregnancies never make it). We must choose the lesser evil in this case and abort, with the knowledge that our sins are forgiven. I sometimes wonder if I should even get out of bed in the morning, because I know it will lead to sin. It is a greater evil to stay in bed and not be fulfilling my vocations as much as humanly possible, so I get up and start sinning.
With that in mind, is the goal of preventing conception always sinful? I say yes. It may sound contradictory, but I also do not claim that there is no potentially acceptable reason when contraception might be the lesser evil. At the same time, I contend that no one has an entirely sinless reason for contraception. Abstinence outside of marriage, of course, is the only God pleasing way to live outside of marriage. But can abstinence ever be sinless within marriage if it is for the purpose of preventing a life from being created by a man and wife? My contention is that it will always be an example of trying to chose the lesser evil. These are rare but difficult decisions that cry out for the aid of one's pastor. But how many pastors understand God's law in the area of procreation? Sadly almost none.
I believe this discussion, thus far, has been missing two absolutely crucial dimensions. First is the biblical explanation of why family planning is sinful. In the post below Pr. Curtis identifies two ways in which contraception can be sinful: goals and means. One way is not having a valid reason (goal?) for contracepting, in which case all means become sinful. The other is having a valid reason, but using a means which is prohibited by God's law, and Pr. Curtis believes NFP is the only valid means.
While I agree that the natural law argument is one of the ways in which Scripture shows family planning to be wrong, we will soon see that there is much more to the "sinfulness" of contraception than this narrow Romanist viewpoint illustrates (no insult to Pr. Curtis intended - I do not believe he is a Romanist). But, alas, the primary discussion of the subject of "why family planning is sinful according to God's Word" must wait for the progression of the discussion on NFP, according to Pr. Rufner's prescription.
THE NATURE OF SIN
The second dimension that I believe has been missing in discussions thus far falls in the context of original sin, which our visitor Lawrence has brought up. In this context we don't deal with means, by which one achieves a goal, but with the very nature of the individual himself. Sin is much more than actions and goals; it is our state of being. This second point is the big picture of the issue which I believe should take precedence.
Let me illustrate what I mean. I do not use any form of contraception, yet I am still guilty of this sin (poor miserable sinner that I am), being contraceptive in my heart. I often desire some form of contraception, even though I use none. It's hard raising six children (though it was harder raising one - not the child's fault, first time parents are a wreck!). My knowledge of God's law regarding procreation curbs me with the first use of the law and instructs me via the third use of the law, but it also always on this earth will convict me via the second use of the law. Praise God for the Gospel!
Many people today believe they are following God's law against adultery because they don't sleep with anyone but their spouse. Christ brings the law into focus by saying: "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart." Likewise, anger is equated with murder. Luther further explains that the perfect fulfillment of this law is more than preventing ourselves from being angry at our neighbor. It is to "help and befriend him in every bodily need."
In the same manner, I believe we need to have the overall view of the sin of contraception as being contraceptive in heart, regardless of what means are, or are not, used. All our sins are forgiven so that we are all perfect saints through faith in Christ, but our sinful nature cannot be reformed so we are always sinners. All things are lawful, but not all things are profitable. That said, what can still condemn us? Only unbelief! Thank God for the gift of faith!
So, why does St. Paul say in 1 Corinthians 6:9 that those who live in manifest sins of the flesh will not inherit the kingdom of heaven? Very simply, and only, because living in such sins (adultery, homosexuality, sodomy, etc.) is incompatible with faith in Christ. It is evidence of unbelief. It should then give one pause that Augustine said contraception was akin to adultery and harlotry. Luther equated it with sodomy. Lukas Osiander said that those who commit it will not inherit the kingdom of heaven.
We all continue to sin daily, but there is a big difference between struggling with sin (of which we repent) and living in sin (which is therefore unrepentant). Nobody goes to hell because of sin. Christ took away the sins of the whole world! People go to hell because of unbelief, which is clearly evidenced by living in manifest sins of the flesh.
Now, you can understand that what concerns me about NFP is that it is not a practice which a couple just happens to fall into, with an accompanying feeling of guilt, like barrier methods can cause. It is the most premeditated and successful way to limit family size known today. It is a complete lifestyle, with daily monitoring of the woman's fertility, with the primary intent of preventing procreation. This is completely contrary to God's law with regard to procreation, and potentially incompatible with faith.
With all the above in mind, consider Luther's explanation of 1 Timothy 2:15, "Notwithstanding she shall be saved in childbearing, if they continue in faith and charity and holiness with sobriety."
"15. 'SHE WILL BE SAVED.' That subjection of women and domination of men have not been taken away, have they? No. The penalty remains. The blame passed over. The pain and tribulation of childbearing continue. Those penalties will continue until judgment. So also the dominion of men and the subjection of women continue. You must endure them. You will also be saved if you have also subjected yourselves and bear your children with pain. 'THROUGH BEARING CHILDREN.' It is a very great comfort that a woman can be saved by bearing children, etc. That is, she has an honorable and salutary status in life if she keeps busy having children. We ought to recommend this passage to them, etc. She is described as 'saved' not for freedom, for license, but for bearing and rearing children. Is she not saved by faith? He goes on and explains himself: bearing children is a wholesome responsibility, but for believers. To bear children is acceptable to God. He does not merely say that bearing children saves: he adds: if the bearing takes place in faith and love, it is a Christian work, for 'to the pure all things are pure (Titus 1 :15).' Also: 'All things work together,' Rom. 8:28. This is the comfort for married people in trouble: hardship and all things are salutary, for through them they are moved forward toward salvation and against adultery.... 'IN FAITH.' Paul had to add this, lest women think that they are good in the fact that they bear children. Simple childbearing does nothing, since the heathen also do this. But for Christian women their whole responsibility is salutary. So much the more salutary, then is bearing children. I add this, therefore, that they may not feel secure when they have no faith." [Luther's Works, Vol. 28, p. 279]
Women (and men) are "not saved for freedom, for license." NFP is a license to sin given out by the Roman Catholic Church. If you have a "valid" reason, Rome will not call you to repentance for valuing your life more than the potential children you are preventing from birth. You have an exception clause! You are sinless in this area of your life. Keep working on the others.
Back to the Lutheran ethical principle of conflicting absolutes. If there is a case where we must choose the lesser evil and prevent the conception of a child, what means should we use? NFP would be a good start, but it is not foolproof (take that word literally). For some, sterilization could be the method of choice. For a hypothetical instance, consider an incurable disease in the wife which would make pregnancy 100% fatal.
If the couple decides that the lesser evil is contracepting, I would recommend a complete hysterectomy for the wife. The husband should not be sterilized, because he may remarry and beget more children. However, if the wife is potentially curable, I would recommend NFP and barrier methods and/or even reversible tubal ligation to greater ensure the life of the mother. All things are lawful, but not all things are expedient. These would be the expedient methods in this case of casuistry, in my understanding of sin.
Much more could (and should) be said, but this post is long enough. More will proceed in comments and further posts.